1. Field of the Invention
The present invention relates generally to an optical coupling circuit for optically coupling an optical waveguide having an optical waveguide forming layer having a lower clad layer, a core layer and an upper clad layer and an optical element mounted on an optical element mounting portion formed by removing a part of the optical wave guide forming layer, and a fabrication process therefor. More specifically, the invention relates to an optical coupling circuit which couples an optical waveguide and a light emitting element, a light sensing element, an optical function element, or the like, and a fabrication process therefor.
2. Description of the Related Art
In general, devices employing an optical waveguide functions not only for transmitting a light beam, but also for distributing an optical power and for synthesizing and dividing wavelength of the light. Therefore, such devices are an component in an optical communication system. However, for application in optical communication, it is not only required to optically couple an optical waveguide of an optical device with an optical fiber, but also to optically couple the optical waveguide with an optical element, such as a light emitting element, a light sensing element, or the like.
Currently, optical communications systems have proposed an fiber network as replacement for a subscriber network, in which electric signal cable in the current subscriber network is replaced with an optical fiber. Accordingly, it is required to extend the optical fiber to each subscriber terminal and to optically couple the optical wave guide, the light emitting element and the light sensing element with the optical fiber line. For this purpose, it is essential to adapt the optical device for mass-production and, of course, to lower cost. In order to realize this, the optical device needs to be down-sized by reducing the number of parts to be assembled, and reducing the number steps for process steps in fabrication. Among these tasks, in the optical coupling of the optical fiber with the light emitting element, a precision typically less than or equal to 1 .mu.m is required. Therefore, such devices are inherently labor intensive. This is the most important impediment for mass-production and lowering of fabrication cost.
The inventors have already proposed a coupling method of an optical waveguide and an optical fiber (Japanese Unexamined Patent Publication (Kokai) No. Heisei 7-36949). In the proposed coupling method, adjustment-free and high precision optical coupling can be performed without complicated processes by forming a V-shaped groove on a waveguide substrate and setting the optical fiber using the V-shaped groove as a guide. However, concerning optical coupling of the light emitting element with the optical fiber or the optical waveguide, since a mode field of the light emitting element is different from those of the optical filer or the optical waveguide, merely less than or equal to one tenth of coupling efficiency can be obtained if the light emitting element with the optical fiber or the optical waveguide are coupled directly. For improving the a coupling efficiency, prior art methods propose inserting a lens between the light emitting element and the optical fiber or the optical waveguide to spatially establish optical coupling, other methods propose processing the tip end of the optical fiber into a hemisphere shaped configuration to provide a lens effect. Furthermore, in these method, since the lens is used, allowable tolerances in alignment error become very small, thereby making it difficult to establish optical coupling in an adjustment-free manner.
However, in the recent years, there has been developed light emitting element having a mode field close to those of the optical fiber and the optical waveguide (for example, Mitomi et al. The Institute of Electronics, Information and Communication Engineers, Electronics Society Conference SC-1-2, 1995) to have about 1 to 2 dB of coupling efficiency and a relatively wide allowable alignment error value of about .+-.1 to 2 .mu.m. In addition, there has been developed a device having a structure in which the optical waveguide and the improved light emitting element are coupled directly each other see, e.g., (Yamada et al. The Institute of Electronics, Information and Communication Engineers, Electronics Society Conference SC-1-11, 1995).
Also, an attempt has been made to perform optical coupling between the light emitting element and the optical waveguide (E. Friedrich et al., Journal of Lightwave Tech., Vol. 10, No. 3, pp. 336-339, 1992). The point of the foregoing technology will be described hereinafter. A plane, to be a reference with respect to the height direction, is required in order to optically couple the optical waveguide and the light emitting element with high precision. In general, upon forming the optical waveguide, by surrounding the circumference of a core layer by a clad layer, absorption of the guided wave by the substrate is prevented and control of the refraction index becomes possible. The distance from the center of the waveguide passage to the surface of the substrate, namely the thickness of the lower clad layer, is required to be greater than or equal to 10 .mu.m. However, the position of an active layer (to be an optical center of a semiconductor laser) employed as the light emitting element is generally 2 to 3 .mu.m from the surface of the element. When the light emitting element is directly mounted on the substrate, coupling cannot be established at all. Therefore, it is necessary to provide a reference plane in the height direction of both of the light emitting element and the optical waveguide. In the method of Yamada et al., unevenness (recess and protrusion) is preliminarily formed on the surface of the substrate. After forming the lower clad layer on the substrate, planarization is performed by polishing the surface of the lower clad layer to expose the protruding portion of the substrate. Thus, a light emitting element mounting portion is provided on the exposed portion of the substrate and a waveguide forming portion is provided at the recessed portion of the substrate where the lower clad layer remains. In this method, since the light emitting element mounting portion is formed at the last step upon removing the waveguide forming layer located thereon, the substrate surface serves as an etching stop layer. Namely, the surface polished and planarized in the preceding step serves as the reference plane in coupling of the optical waveguide and the light emitting element. Furthermore, the exposed substrate surface serves as a heat sink for the light emitting element upon mounting the light emitting element.
On the other hand, in the method of E. Friedrich et al, upon removing the waveguide forming layer at the light emitting mounting portion, etching of part of the mounting portion is stopped at a position where the height of the optical waveguide and the light emitting element becomes optimal. By taking this surface as the reference plane, the optimal coupling between the light emitting element and the optical waveguide can be realized.
Also, in Japanese Unexamined Patent Publication No. Heisei 5-100122 by Okamura et al., a thin film layer is formed in the vicinity of the core layer. The upper surface of the thin film layer is exposed as the reference plane. By tightly fitting this reference plane with the reference plane of the guide member supporting the optical element, coupling of the optical element and the optical waveguide is established.
However, the above-mentioned prior art encounters the following problems. Namely, in the method of Yamada et al, the exposed surface of the substrate is taken as the reference plane in the height direction of the optical waveguide and the light emitting element. Therefore, high precision alignment becomes possible only when, polishing is in performed the fabrication process in order to provide the reference plane. This makes the process complicated, and therefore, unfit for mass-production. In case of these devices mass-production of these devices, it is ideal to perform fabrication only by steps of layer formation, patterning, etching and so forth in batch process (as in the semiconductor LSI process).
On the other hand, by the method of E. Friedrich et al., in order to obtain high precision alignment, etching must be stopped at the optimal position. Therefore, it is necessary for precision control of the etching speed of the etching device. Particularly, in mass-production, since a large number of optical coupling circuits are formed on one wafer, it is necessary to accurately control distribution of etching speed in the wafer and distribution of layer thickness of waveguide forming layer to be etched. These problems are quite difficult to solve. Therefore, it is not practical to mass-produce the device employing this method.
Also, in the method of Okamura et al,. since a guide member is required for the optical element to be coupled with the optical waveguide, the number of necessary parts becomes large. Furthermore, when the optical element is directly mounted on the substrate employing this method, since an under-clad is present between the substrate and the mounting position, while it is effective for coupling the element not causing heat, such as the optical fiber, the light sensing element and the like and the optical waveguide, it is not effective for coupling the light emitting element which generates a heat, as sufficient heat radiation cannot be performed.
In order to enable mass-production of the optical coupling circuit for the optical waveguide and the light emitting element toward the future, it is essential to enable fabrication process, to stably form the reference plane in the height direction in the wafer, and to permit sufficient heat radiation of the light emitting element.